An investigation into the causes for the reduction in the variability of the El Niño-Southern Oscillation in the early Holocene in a global climate model

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Abstract

A large body of evidence shows that during the early Holocene the variability on interannual timescales in the tropical Pacific was much reduced in comparison to today. We investigate a possible cause for this using output from a Global Climate Model forced by present day and early Holocene boundary conditions, and a simplified model that is known to simulate the El Nino-Southern Oscillation (ENSO).We devise a scheme to incorporate mean state data, into the background states of a linearisation of the Zebiak and Cane model (Zebiak and Cane, 1987), due to Thompson and Battisti (2000). We show that when sets of background states are compiled using observations of the present climate that ENSO mode in the linearised model is stable.Using the method for incorporating mean state data into the linearised coupled model on output from the Community Climate System Model (Otto-Bliesner et al., 2003) run with present day and 8.5 thousand years before present (ka) forcing we find that the mean state change in the tropical Pacific at 8.5ka is sufficient to explain the reduction in the variance of the model's nino3 index. We show that changes in the ocean are not responsible for increasing the stability of the ENSO mode, and decreasing the variance of the modelled nino3, but are responsible for changing the pattern of the ENSO variability at 8.5ka. We show that the general cooling in the sea surface temperature at 8.5ka, compared to today, results in a reduction in the windstress variance which acts to stabilise the ENSO mode, and so reduce the variance of nino3 at 8.5ka.